The invention relates generally to dye transfer printers such as thermal printers, and in particular to the problem of crease formation in the dye transfer area of a donor web used in the printer. Crease formation in the dye transfer area can result in an undesirable line artifact being printed on a dye receiver.
A typical multi-color donor web that is used in a thermal printer is substantially thin and has a repeating series of three different color sections or patches such as a yellow color section, a magenta color section and a cyan color section. Also, there may be a transparent laminating section after the cyan color section.
Each color section of the donor web consists of a dye transfer area that is used for dye transfer printing and pair of longitudinal edge areas alongside the transfer area which are not used for printing. The dye transfer area is about 95% of the web width and the two edge areas are each about 2.5% of the web width.
To make a print, the various color dyes in the dye transfer areas of a single series of yellow, magenta and cyan color sections on a donor web are successively heat-transferred by a print head onto a dye receiver such as paper or transparency sheet or roll. The dye transfer from each transfer area to the dye receiver is done line-by-line widthwise across the transfer area via a bead of selectively heated resistive elements on the print head. The print head makes line contact across the entire width of the color section, but it only heats the dye transfer area, i.e. it does not heat the two edge areas alongside the dye transfer area.
As each color section is used for dye transfer at the print head, the donor web is subjected to a longitudinal tension between a donor supply spool and a donor take-up spool which are rearward and forward of the print head. The longitudinal tension, coupled with the heat from the print head, causes a used color section to be stretched lengthwise at least from the print head to the donor take-up spool. Since the dye transfer area in a used color section has been heated by the print head, but the two edge areas alongside the transfer area have not been heated, the transfer area tends to be stretched more than the edge areas. As a result, the transfer area becomes thinner than the two edge areas and develops a wave-like or ripple distortion widthwise between the edge areas.
After the last line is transferred from a dye transfer area to a dye receiver, and as the used color section is advanced forward from the print head and onto the donor take-up spool, the wave-like or ripple distortion in the transfer area causes one or more creases to form at least in a short trailing or rear end portion of the transfer area that has not been used for dye transfer. The creases tend to spread rearward from the trailing or rear end portion of the used transfer area into a leading or front end portion of an unused transfer area in the next (fresh) color section being advanced to the print head. The creases appear to be created because of the difference in thickness between the used transfer area and the edge areas as they are wound under tension from the print head and onto the donor take-up spool.
When a used color section is wrapped under tension around the donor take-up spool, the edge areas wrap differently on the spool than does the used transfer area because of the difference in thickness between the transfer area and the edge areas. As each additional color section is wrapped around the donor take-up spool, the convolution build-up of the thicker edge areas on the spool becomes significantly greater than the convolution build-up of the thinner transfer areas. This non-uniform winding of the used color section increases the likelihood of one or more creases being created because the convolution build-up of the thicker edge areas on the donor take-up spool adds to the tension and distortion of the used transfer areas.
A problem that can result is that a crease in the leading or front end portion of the unused transfer area of the next (fresh) color section will cause an undesirable line artifact to be printed on a leading or front end portion of the dye receiver when the print head is applied to the crease. The line artifact printed on the receiver is about 0.5 inches in length.
The question presented therefore is how to solve the problem of the creases being created in the unused transfer area of each fresh color section so that no line artifacts are printed on the dye receiver.
A dye transfer printer in which a donor web having successive dye transfer areas that are used for printing and opposite longitudinal edge areas alongside each one of the dye transfer areas that are not used for printing are wrapped under longitudinal tension about a cylindrical spool core, and in which the dye transfer areas that are wrapped under tension about the spool core have been stretched thinner than the two edge areas that are wrapped under tension about the spool core, is characterized in that:
the cylindrical spool core is depressible inward at respective portions on which the two edge areas are wrapped under tension about the spool core and is not depressible at an intermediate portion between the depressible portions on which the thinner transfer areas are wrapped under tension about the spool core, whereby a convolution build-up of the edge areas on the spool core will be no more than a convolution build-up of the thinner transfer areas on the spool core.
If the convolution build-up of the edge areas on the spool core is no more than the convolution build-up of the thinner transfer areas on the spool core, the likelihood of the creases being created in the unused transfer area of each fresh color section is substantially reduced. Thus, no line artifacts can be printed on the dye receiver.